It has long been recognized that long straight chain paraffin hydrocarbons containing upwards of about 18 carbon atoms will crystallize from a solution in petroleum hydrocarbons at substantially lower temperatures than the freeze-point of other hydrocarbons of like boiling point. A fraction separated from a waxy crude oil by distillation will become incapable of flow from a vessel at a temperature (the pour point) such that the wax crystals formed will inhibit such flow. Liquid fuels cannot be used in the intended manner at temperatures below the pour point. Difficulties due to poor pumpability and clogging of filters can be encountered at higher temperatures due to suspended wax crystals in the soil.
Dewaxing of hydrocarbon oils has been practiced for many years by chilling the oil, usually in a solvent, and separating the wax crystals, as by filters, centrifuges, and the like. A more recent development for reducing the pour point of hydrocarbon fractions is catalytic hydrodewaxing in which a mixture of hydrogen and waxy hydrocarbon fractions is contacted at conversion conditions of temperature and pressure with a shape selective porous solid catalyst having acid activity for cracking in combination with a metallic hydrogenation/dehydrogenation catalyst. The porous solid catalyst is characterized by uniform pores which will admit only straight chain or straight and slightly branced chain aliphatic compounds and therefore converts only those compounds so admitted. Typical dewaxing catalysts include crystalline zeolites which are shape selective, i.e., capable of selectively cracking the waxy paraffinic constituents of the feed without excessive cracking of larger moclecules. Such zeolites are characterized by a constraint index of 1-12. In some instances, the dewaxing catalyst may be used without hydrogenation/dehydrogenation metal functions.
In addition to catalytic hydrodewaxing which is a shape selective cracking process, heavy hydrocarbon stocks, particularly those containing sulfur, nitrogen, and metal contaminants have been converted to provide good yields of such premium products as motor gasoline, diesel fuel, jet fuel, distillate fuel oil, and kerosene as well as heavier (higher molecular weight) products suitable for blending with the premium products by catalytic cracking or hydrocracking the feedstock to lower molecular weight materials to reduce the boiling point of the constituents of the heavy stocks. The sulfur content of heavy fractions from many crudes, however, exceeds environmentally acceptable limits. This characteristic of heavy crude fractions is usually handled by hydrodesulfurization, a catalytic reaction under hydrogen pressure in the presence of a catalyst having hydrogenation/dehydrogenation activity such as cobalt and molybdenum oxides or sulfides on a refractory support such as alumina.
Thus, it has become common practice to hydrotreat certain stocks for removal of sulfur, nitrogen, and metals. For example, feed for hydrocracking may be first contacted with a hydrotreating catalyst as discussed above in the presence of hydrogen. The hydrotreater effluent is condensed and separated from unused hydrogen, ammonia, hydrogen sulfide and gaseous hydrocarbons such as methane for recycle to the reactor after scrubbing to remove hydrogen sulfide and ammonia. The condensate is then mixed with a further supply of hydrogen and passed through one or more beds of hydrocracking catalysts to produce products of lower boiling range than the feed. Similarly, a catalytically hydrodesulfurized chargestock can be shape selectively hydrocracked, i.e., dewaxed. Further, it has been proposed to simultaneously dewax and desulfurize a resid at conventional resid desulfurization conditions using a combined hydrotreating/ZSM-5 zeolite catalyst.
What has been found, however, is that when the single-catalyst systems such as just described above is used simultaneously for hydrotreating and dewaxing an atmospheric resid, a low sulfur 775.degree. F.+ bottom fraction and a low sulfur, low pour point 650.degree.-775.degree. F. fraction are produced that could be blended to the refinery distillate pool, but the kinematic viscosity of the 775.degree. F.+ bottom fraction is excessively high making this bottom fraction unsuitable for a heavy fuel oil application. A more expensive cutter stock must be used for blending, but such use of blending stocks penalizes the process economics. Furthermore, it was found that the pour point reductions of the 380.degree.-650.degree. F. and 650.degree.-775.degree. F. fractions were much greater than the reductions required by product specifications. Moreover, the 380.degree.-650.degree. F. and 775+.degree. F. fractions from a conventional hydrodesulfurization unit without dewaxing function have been found to meet the product specifications. Thus, it would appear desirable to dewax the 650.degree.-775.degree. F. fraction alone.
Accordingly, it is an object of the present invention to upgrade a petroleum resid by desulfurizing such resid and subsequently treating the 650.degree.-775.degree. F. fraction alone, thus leaving undisturbed hydrocarbon distillate boiling below 650.degree. F. and above 775.degree. F.